The present invention relates to power supplies, and more particularly, to power supplies for outputting a preselected one of a plurality of regulated DC output voltages.
In many electronic systems, and in particular in data processing systems, there is always the need for providing regulated DC voltages to the logic of the system. Specifically, the power requirements generally include providing a plurality of voltage levels. For the sake of economics in manufacturing, field spares, . . . , it is desirable to have one power supply assembly that is operable over all the required voltages. The power stage of a power supply, or sometimes referred to as a switching regulator, is generally adaptable to such versatility. In present existing power supplies, the output voltage of a switching regulator is sometimes modified through the use of resistive voltage dividers in an output voltage sensing circuit. By feeding back a selected portion of the output voltage, it is possible to increase the output voltage over that obtained when the full output voltage is fed back. However, it is not possible to reduce output voltage below the initial design value by this method. The control circuit associated with the power supply is not always readily adaptable, especially at low output voltages. Switching regulator type power supplies regulate the output voltage by comparison with a fixed reference voltage. Thus, it is possible to alter the control circuit in a number of ways to obtain different values of regulated voltage. The obvious approach is to change the reference voltage to the desired value. However, in existing power supplies this is generally not feasible. In the typical implementation of a control circuit, ther is included a number of integrated circuits that utilize the sense voltage, i.e., the output voltage, as a supply or source voltage. These circuits may not be operable from such lower source voltages. The same applies to circuit elements incorporated in an over-voltage protection circuit, and in other "housekeeping" circuits such as under-voltage detection circuits, etc. Furthermore, if a regulator is to be selectively operable at more than one output voltage, all such circuitry must be modified by a variety of means, including switches, connectors, jumpers, . . . .
Therefore, there exists a need to provide a power supply which can provide one of a plurality of regulated output voltages, the regulated output voltage being selectable without requiring extensive circuit design changes or numerous changes in component values. The power supply of the present invention adds a selectable gain amplifier to the power supply feedback loop. The selectable gain amplifier of the preferred embodiment is provided with a plurality of feedback resistors and elements for selectively connecting them into the amplifier circuit thereby varying the gain of the amplifier.